Process for roasting ores



Patented Apr. s, 1938 PATENT OFFICE raocEss roaaoAs'nNo oaEs Edwin J.Mullen, New Rochelle, N. Y., assignor to General Chemical Company,NewYork, N. Y., \a corporation oi' New York Application November 26,1937, Serial No. 176,534

-10 Claims.

This invention is directed to methods for roast-V ing sulfide ores todesulfurize the same and to produce sulfur dioxide for use in themanufacture of sulfuric acid, or for any other purposes de- 5 sired. Theinvention is a development of my co-pending applications Serial Nos.55,808 and 55,809, filed December 23, 1935.

To a substantial extent, present practice in roastingv of sulfidessuchas pyrites includes use l0 naces, such for example as the well-knownMac- Dougall, Herreshoff and Wedge burners. As distinguished from the'bed vor hearth vroasting operation of these burners, it has beensuggested to roast finely divided sulfldes while in gaseous suspension.Suspension roasting processes such as shown for example in Burgoyne andCordy U. S. Patent No. 1,758,188 of May 13, 1930 have been developedlargely because of availability in re- U cent years of supplies ofsulfldes, such as flotation concentrates, sulciently' nely dividedtopermit roasting by suspension methods.

In the present state of the art of producing sulfur dioxide fromsulfldes, it may be said that probably the major portion of the sulfurdioxide utilized in the manufacture of sulfur trioxide by the contactprocess is obtained by roasting what is known as pyrltes smal1s. Theusual run-ofplle of pyrltes smalls has been crushed so that 100% passesabout a half inch screen. A sub- :tantial portion of an ore of this typeis sufficiently nely divided for suspension roasting if separated fromthe coarser material, although a large portion of the smalls, in manycases the V5;-, major portion, comprises ore too coarse for Suspensionroasting by present methods. In the present specication, smalls is usedin a generic sense to define a run-of-plle ore containing some ore fineenough for suspension roasting and some ore too coarse for suspensionroasting. The term coarse is used to denote ore of too large parti# `clesize for roasting by suspension methods, and fines is utilized todesignate sulfide ores sufficiently finely divided to permit roasting bysuspension methods.

Since smalls usually contain a substantial portion of coarse ore, thesmalls have heretofore been roasted as in a multi-hearth roaster. Whilemulti-hearth furnaces such as the Wedge and Herreshoff burners provideeffective roasting, use of burners of this type is objectionableprincipally on account of the low capacity per unit of apparatus perunit of time and because the complicated construction and operationinvolves substantial initial and maintenance expense. On

of mechanically operated multiple hearth furthe other hand, grinding ofsmalls by usual grinding operations, prior to roasting, to such degreeof subdivision that all of the ore may beroasted in gaseous suspensionis not desirable since, as

.well-known, fine grinding is one of the most expensive operations inmetallurgical processes and in many instances grinding costs areprohibitive.

As disclosed in my earlier applications, I have previously found thatwhen coarse ore is introduced into a combustion zone and suddenly subjected to certain temperatures, preferably'not less than about 1300o F.,the coarse ore particles are converted to a frangible, brittle conditionwhich may be carried out in simple, rugged ap# paratus readilyconstructed and economically maintained.

The nature of the invention and the objects and advantages thereof maybe fully understood from consideration of the following descriptiontaken in connection with the accompanying drawing illustrating invertical longitudinal section a burner in which the improved process maybe carried out. Referring to the drawing, I0 designates a burnercomprising a steel shell or casing ll within which is placed the furnacelining I2 constructed of suitable refractory material, such asiirlebrick and dening a roasting chamber I4 of circular, horizontalcross-section.` The upper part of the. combustion chamber is closed'offby a crown I5 the top side of which forms a drying or preheating hearthI'I. 'I'he shell II projects upwardly beyond crown I5 and carries asteel framework i8 which in turnsupports ore feeding and rabblingmechanism for the drying hearth. The

-surface of hearth I1 is slightly cone-shaped and slopes downwardlytoward the shell of the burner.

Lying above the hearth arerabble arms I9 having downwardly projectingplows 20 pitched to work suldes gradually toward the circumference ofthe drying hearth. Arms I9 are rotated by motor 22 through shaft 23supported in bearings so as to maintain the lower ends of plows 20-spaced with respect to the surface of hearth I1.

An ore bin 24, mounted on framework I8, disl charges ore onto a platform25 from which the from shaft 63 through gear 64.

smalls are intermittently dropped to the center of drying hearth I1 by asweep 21 rotating with shaft 23.:l

Cut through shell and also through the up- .per edge of lining I2 is a.passage 34 through which smalls are passed from hearth I1 into a conduitthru which the smalls are transferred to hopper 36. On rotation ofrabble arms I9, the sulfide ore is gradually fed through opening 34 intoconduit 35.

The bottom of the combustion chamber is formed by a cone-shaped hearthor hopper 40, of substantial vertical length, terminating in a cinderdischarge opening 42 which may if desired be provided with a gas lockmechanism through which cinder may bevdischarged without permitting gasto escape from the combustion chamber.

Part of the air needed to effect roasting is drawn into chamber |4 thruports 45, 46, and 41, each provided with a damper or door 49 by means ofwhich the amount of entering air may be regulated.

Numeral 55 indicates a grinder-burner com-f prising a steel shell 56having a suitable refractory lining 51 and-forming a preliminarygrinding-roastlng chamber 58. Furnace 55 is rotatably mounted onbearings 60 and 6| and is driven One end of furnace 55 is provided witha fixed head 66, supported by framework not shown and arranged inrelatively gas-tight relationship with the end 'of the shell 56. Theopposite end of furnace 55 projects through opening 68 in the lowerportion f ol the vertical wall of the combustion chamber I4. Smalls arefed into furnace 55 from hopper 36 through conduit 31 controlled byvalve 38.

pheric air into pipe 15 by means of an air inlet l f 11 having aregulating damper 18. Air-line 80 is connect-cd at one end to the outletside of blower` 16 and projects thru fixed head 66 into the grinding andpreliminary combustion chamber 58.

A gas main 85 for withdrawing gaseous products of combustion from theburner opens into combustion chamber I4 at a point just below crown I5.Main 85 conducts such gases into a waste heat boiler 81 in which steamdrum 89, headers 90, and water tubes 9| may be of standard construction.Gases from combustion chamber I4 are discharged through line 85 into achamber 94, formed partly by vertical lire-wall 95, in which dustentrained in the gases settles out and collects in hopper 91 preferablyprovided with an air-lock 98 `through which dust may be withdrawnwithout admitting air to chamber 94.- After contacting water tubes 9|the gases pass downwardly through chamber |00 in which further settlingout of entrained solid particles is effected. Chamber |00 is likewisesimilarly provided at the bottom with` an air-lock discharge |02.Movement of the gases through the system is largely controlled by ablower. not shown, to the inlet side of which gas-line |03 is connected.

The invention is applicable to the roasting of sulfide ores such as ironpyrites, pyrrhotite, zinc sulfide or arsenopyrlte, but for convenienceoperation of the process will be described in connection with theroasting of iron pyrltes smalls.

In the following discussion it may be assumed the ore referred to is arun-of-pile or pyrites "smalls" ore, 100% passing a. half inch screenand containing about 50% coarse ore insufficiently finely divided forsuspension roasting, and about. 50% fines le. g. 30 mesh or finer) ofsuch size as to permit roasting in suspension.

A supply of pyritcs smalls is maintained in bin 24 by suitable conveyoror elevator, not shown. Before roasting is begun, grinding andpreliminary roasting chamber 58 and combustion chamber I4 are preheatedto temperatures above the ignition point of the ore to be roasted, as byoil burners inserted through conveniently located work-holes .not shown.When the desired degree of preheat is obtained, motor 22 is started, andrabble arms I9 and sweep y21 may be rotated at a rate of say onerevolution in two minutes. Smalls run continuously out of bin 24 ontoplatform 25, and on each revolution of shaft 23 a regulated quantity ofore is swept off the platform to approximately the center of dryinghearth I1. During rotation of arms I9, the sulfide smalls are graduallyworked across the surface of hearth I1, into passage 34, and thence thruconduit 35 into hopper 36. During movement of the smalls over hearth I1,the fines and the coarse ore are thoroughly dried and may be preheatedas a rule to about 250 F. and generally not in excess of about G-500 F.

When starting up the process, in addition to preheating preliminarygrinding and roasting chamber ,58, and suspension roasting chamber I4 totemperatures in excess of the ignition t em` perature of the particularsulfide ore being treated, chamber 58 is preheated to temperatures notless than about 1300 F. As will hereinafter appear', during course ofoperations the extent of combustion of sulfur in chamber 58 iscontrolled so that such chamber is maintained at temperatures preferablynot less than about 1300 F. Owing to the existence of such temperaturesin chamber 58 the coarse ore particles charged thereto are very suddenlysubjected to relatively high temperatures. Experience indicates theloosely combined sulfur of the coarse ore is practically instantaneouslyvolatillzed on sudden introduction of the coarse ore into the hightemperature atmosphere in chamber 58, and on account of such suddenexposure to these temperatures and consequent differential expansion,strains are set up in the coarse ore particles thus disrupting andconverting the ore particles to a porous condition. When the coarse oreis introduced suddenly into chamber 58 the resulting differentialexpansion and rapid volatilization ofthe loosely combined sulfur appearsto exert within the lumps of the coarse ore. explosive forces which openup the ore rendering the particles porous and converting the same to afrangible condition offering very little resistanee to crushing. It willbe appreciated that this makes possible easier and more economicalgrinding'of the coarse fraction and greatly simregulation of blower 18,damper 12 in the prenot in excess of about 40o-500 TF. and collecting inhopper 38 run through conduit 31 into chamber 58, rate of feed ofsulildes being controlled by a suitable valve Il. As explained above,when these coarse ore particles are suddenly subjected ore to a.particle size suitable for. suspension' roasting is very rapid.

In the present example approximately half of.

the air needed to eil'ect oxidation of the total sul-` iur and the ironof the initial ore is fed into lchamber 58 through pipe 88. Appreciableroasting of both fines and coarse ore takes place, such roastinghoweverprobably comprising principally oxidation of loosely combined sulfur ofboth lines and coarse ore. Part `or all of the air needed in chamber 58may be drawn into the system through the inlet 1I in the lower end ofair preheating jacket 1I)V surrounding the suspension Y process arecontrolled so as to maintain in chamber 58 temperatures of not less thanabout 1300 F., and passage of a gas current of sufficient `velocity tosweep up and carry out of chamber 58 and into suspension roastingchamber I4 those particles which are suillciently fine, e. g., 30-35mesh and smaller. to permit roasting in gaseous suspension. l

The present improvements relate primarily to suspension roasting and itis the object to eiect as much roasting as possible in suspension andthereby make use of the economic advantages of suspension roasting,principally simple apparatus. Hence, the purpose of grinder-burner 55 isonly to eilect subdivision of coarse ore to suspension roasting particlesize, and no particular advantage is to be gained by maintainingtemperatures in chamber 58 much in excess of i300-1400 F. To obtain thebest economic advantages, preferably temperatures should not bepermitted to appreciably exceed l550 because at higher temperaturesfusion (conversion to magnetic FeaOl) begins to take place. Fusedparticles are hard and not `readily grindable, and hence formation ofappreciable amounts of fused particles defeats the primary purpose(conversion to readily grindable form) of the coarse ore treatment ingrinder-burner 55. When proceeding so as to maintain the temperaturesdescribed tests show that-the coarse ore particles for the most part areso frangible aste be crushable by pressure of the hand, this indicatinghow little attrition and .consumption of power are needed to pulverizethe coarse ore. tenance of proper temperatures, the extent of sul'- iurcombustion, the rapidity of grinding of the frangible coarse ore, andthe velocity of the gas current in chamber 58 may be readily obtained bycontrol of feed of smalls from hopper 36, rate of rotation of shell 56,and temperature and amount of air introduced into chamber 58 throughpipe 80. It will be appreciated that by The mainheating jacket air inletpipe 1I, and damper 18 in cold'air inlet pipe 11, the quantity andtemperature of the air entering chamber 58 may be very closelycontrolled. The result of operation of ,the preliminarygrinding-roasting phase of the process is such as to quickly and cheaplyreduce all of the coarse ore particles to suspension roasting particlesize and to etlect some roasting oi.' all of the ore with consequentproduction of some sulfur dioxide. No particular economies are to begained by conducting operations in chamber 58 in any way other than toaccomplish these ends.

A substantial operating advantage afforded by the preliminary oxidationtaking place in chamber 58 is that since substantial quantities ofsultur are burned in chamber 58 corresponding amounts of heat aregenerated most oi.' which heat is dissipated by radiation from the wallsof the rotating shell 58. This leaves less heat to be generated in thesuspension roasting zone with the result that the tendencies to slag upand agglomerate the suspension roasting zone cinder are appreciablyreduced.

The velocityof gas movement through chamber 58 and suspension roastingzone I4 are controlled by the blower, not shown, in gas main |03, andthe auxiliary blower 18 ahead .of the grinderburner 55. By regulation ofthese two blowers velocity of gas movement to chamber 58 is controlledso as to be just suillciently high to sweep` more injectors 13 a smallpart of, the air to be charged into chamber I4. Such air would aid incarrying the lines particles to the top of chamber I4.

The reason for limiting the amount of air entering chamber 58 is tofacilitate maintenance therein of temperatures only suillclently high tobring about the described relative explosion of the coarse oreparticles, and not to effect any this purpose. In the'present example,using the particular ore comprising about @50% coarse ore and 50% fines,lroughly about -half of the total amount of air required to combust allof the ore is introduced into chamber 58. Hence,v further amounts of-airor other oxidizing gas are needed to effect roasting, vand such air isdrawn into the suspension roasting zone I4 through vports 45, 46, and41. The rate of movement of the upwardly flowing gas stream in chamberI4 is such that the ore particles are carried upwardly well toward thetop of chamber I4. However, sincethe rate of gas movement upwardly trough chamber I4 is appreciably less, on account f the relatively largecross-section ofchamber I4,'than the velocity of the gas stream passingthru chamber 58, the fines particles soon lose their initialmomentum'and for the most part drop more or less vertically throughchamber I4, dotted line |05 on the drawing 4indicating the generalcourse of a fines particle through chamber I4.

On introduction of the fines into chamber I4 flash vroasting of` the nescommences and/or further combustion of sulfur than is necessary forAl..A

increases with o great rapidity. relatively. large amounts of heat aregenerated, and ,during the upward and downward course of travel oi.' theilnes particles through chamber I4 roasting proceeds to completion. Gastemperatures prevailing in chamber I4 are comparable with those existingin known suspension roasting methods, e. g., of the order of 180o-2000F. In general, the roasting of the fines particles in chamber I4 issimilar to that disclosed in my U. S. Patent No. 2,070,236, o i February9, 1937. The resulting iron oxide cinder drops onto hearth 40 and isdischarged from the furnace as required.

In the modification illustrated in the drawing, opening 68 into .whichthe outlet end of grinderburner' 55 projects is relatively in the lowerend of the suspension roaster. Gas movement in chamber I4 is in anupwarddirection, the sulfur dioxide gases being withdrawn from the topoi' the combustion zone. I f desired, opening 60 may be adjacent the topof chamber I4 and grinderburner 55 mounted in a similarly elevatedposition. In such modified apparatus, air inlets corresponding withports 45, 46, and 49 may be 1ocated in the upper periphery ofthe wallsot chamber I4 just beneath crown I5. Sulfur dioxide gas outlet 05 wouldthen be located near the bottomI of the combustion chamber. In thismodication, the travel of the lines and gas in chamber I4 would be in adownward direction only and the suspension roasting-phase wouldv be aco-current operation similar to that shown in Burgoyne et al. patentpreviously mentioned. l The sulfur dioxide gases produced .enter wasteheat boiler 81 at temperatures of around 1800- 2000 F. Heat is recoveredin the form of steam and entrained dust settles out and'v collects inchambers 94 and |00. The gas stream enters line |03 at temperatures ofsay 50B-600 F., and may be purified if desired for use, e. g. in themanufacture of sulfuric acid bythe contact process.

The invention has been described in connection with roasting of smallscontaining about 50% coarse ore and 50% lines because the usual run ofsmalls contains coarse ore and fines in approximately these proportions.It will be understood, however, that the process of the invention isreadily adaptable to handle smalls containing less or greater quantitiesof coarse ore. In the case of smalls containing less amount of coa-rseore, the smalls may be fed to grinderburner 55 at a greater rate,and-where the smalls contain greater quantities of coarse ore rate ofintroduction of smalls may be correspondingly reduced. Regulation ofcontrol conditions in accordance with the nature of the smalls used willbe apparent to the skilled operator. The invention presents severalsubstantial operating advantages. For example, when working with pyritessmalls passing a half inch screen as mentioned, it would ordinarily benecessary to roast this type of ore in a multi-hearth burner or go tothe expense of grinding the ore to a sufficient degree of fineness topermit roasting bysuspension methods. In accordance with the presentprocess, on account of the exceedingly frangible condition to which thecoarse ore is rapidly converted, whatever grinding is necessary may beaccomplished with much less expenditure o! power than would be the caseif the o coarse ore constituents were pulverized in usual grindingoperations.v By the present method,

' smalls maybe roasted in a way much more eilicient than could beaccomplished in a multiu hearth roaster. An outstanding advantageafforded by the invention is that operations may b e carried out insimply constructed and ecov nomlcally maintained apparatus.` l'

I claim:

1. The method for roastingl coarse metal sulde ore which comprisesmaintaining avzone at temperatures not less than about 1300 F.,introducing the coarse ore into said zone whereby the ore is suddenlysubjected to temperatures of the combustion zone and converted to arelatively porous frangible condition, subjecting the ore while in saidzone to attrition suiicient to reduce the coarse ore to nnes, thenintroducing said ilnes into a combustion zone, forming therein asuspension of lines in oxidizing gas, roasting the fines while insuspension in the oxidizing gas to produce sulfur dioxide, andrecovering sulfur dioxide.

2. 'I'he vmethod for roasting coarse metal sulfide ore which comprisesmaintaining an oxygen-containing combustion zone at temperatures notless than about 1300 F.,introducing the coarse ore into said zonewhereby the ore is suddenly subjected to temperatures of the combustionzone, converted to a relatively porous frangible condition and partiallyroasted, subjecting the ore while in said'zone to attrition sufficientto reduce the ore to fines, then introducing said fines into a secondcombustion zone, forming therein a suspension of fines in oxidizing gas,roasting the fines while in suspension in the oxidizing gas to oxide.

3. The method for roasting coarse metal sulfide A ore which comprisesintroducing the coarse ore into an oxygen-containing combustion zoneheated to temperatures not less than about 1300 F. whereby the ore issuddenly subjected to temperatures of the combustion zone, converted toa relatively porous Irangible condition and partially roasted,maintaining such temperatures by at least partial combustion of some ofsaid ore, subjecting the ore while in said zone to attrition sufcient toreduce the ore to lines, then introducing said nes into a secondcombustion zone, forming therein a suspension of iines ,in oxidizinggas, roasting the fines while in suspension in the oxidizingl gas toproduce sulfur dioxide, and recovering sulfur dioxide.

4. The method for roasting metal sulfide smalls containing coarse suldeore and initial sulfide fines which comprises maintaining anoxygencontaining combustion zone at temperatures not `less than about1300 F., introducing the smalls `containing coarse sulildeore andinitial sulde nes which comprises introducing the smalls into anoxygen-containing combustion zone heated to temperatures not less thanabout 1300 F. whereby 'the smalls are suddenly subjected to temperaturesof the combustion zone and the coarse ore is converted to a relativelyporous frangible condition and partially roasted, mainroasting the fineswhile in suspension in the oxi-l dizing gas to produce sulfur eringsulfur dioxide.

6. The v method for roasting metal sulfide dioxide, and recovsmallscontaining coarsesuliide ore and initial sulfide nes which comprisesmaintaining-an oxygen-containing combustion zone at temperatures notless than about 1300 F. and not substantially in excess of about 1550F., introducing the smalls into said zone whereby the smalls aresuddenly subjected to` temperatures of the combustion zone and thecoarse ore is converted to a relatively. porous frangible condition andpartially roasted, sub- `iecting the coarse ore while in said zone toattrition sufiicient to reduce the same to iines, then introducing suchresulting iines together with `said initial nes into a second combustionzone,

forming therein a suspension of fines in oxidizing gas, roasting the neswhile in suspension in the oxidizing gas .to produce sulfur dioxide, andrecovering sulfur dioxide.

7. The method for roasting metal -sulilde smalls containing coarsesulilde ore and initial suliide` lines which comprises introducing thesmalls in' to a combustion zone heated to temperatures not less thanabout 1300 F. whereby the smalls are suddenly subjected to temperaturesof the combustion zone and the coarse ore is converted to a relatively'porous frangible condition and partially roasted, passing through saidzone a gas stream containing oxygen in amount less than that needed tosupport complete oxidation of said smalls but in quantity suilicient tomaintain such temperatures by at least partial combustion of some ofsaidsmalls, subjecting the coarse ore while in said zone to attritionsuiiicient to reduce the same to ilnes, regulating ,the velocity of thegas stream passing through said zone so as to sweep out of said zonefines resulting from such attrition together with said initial fines,introducing the gas stream containin-g such iines into a secondcombustion zone. forming therein a suspension of `lines in oxidizing gasin amount suillcient to eil'ect substantially complete oxidation of saidnes, roastingthe fines while in suspension in the oxidizing gas toproduce sulfur dioxide, and recovering sulfur dioxide.

8. The method for roasting metal sulilde smalls containing coarsesulfide ore and initial sulde lines which comprises introducing thesmalls into a combustion zone heated to temperatures not less than about1300 F. whereby the smalls are suddenly subjected to temperatures of thecombustion zone and the coarse ore is converted to a relatively porousfrangible condition and partially roasted, passing through said zone agas stream containing oxygen in amount less than that needed to supportcomplete oxidation of said 'smalls and in quantity restricted tomaintain such temperatures not substantially in excess of .about 1550F., subjecting the coarse ore while in said zone to attrition suiiicientto reduce the same to lines. maintaining the velocity-of the gas streamthrough said zone high lenough but not substantially in excess of thatrequired to sweep out of said zneviines resulting from l such attritiontogether with said initial fines. in-

troducing the gas stream containing such fines a suspension oi' fines inoxidizing gas in amount sufficient to eilect substantially completeoxidation oi' said lines, roastingy the nes while in suspension intheroxidizing gas to Produce sulfur di oxide, andrecovering sulfurdioxide. y

9. The method for roasting metal sulfide smalls containing coarsesulilde ore and initial i into a second combustion zone, forming thereinsulde Viines which comprises introducing the of the combustion zone andthe coarse ore is Y converted to a relatively porousfranglble conditionand partially roasted, passing through saidv zone a gas streamcontainingoxygen in amount less than that needed to support completeoxidation of said smalls but in quantity sumcient to maintain suchtemperatures by at least partial combustion of some of said smalls,subjecting the coarse ore while in said zone to attrition sutiicient to-reduce the same to nes, regulating the velocity of the gasstreampassing through said zone so as to sweep out of said vzone iinesresulting from such attrition together with said initial'nes,introducing the gas stream containing such lines into the lowerportion.v of a second combustion zone. forming therein a. suspension voffines in an upwardly-'moving stream of gas containing oxygen in amountsumcient to effect substantially complete oxidation of saidnes,

smalls containing coarse sulilde ore and initial sulfide lines whichcomprises introducing the smalls into a combustion z 'one heated to*temperatures not less than-about l300 F. whereby the smalls are suddenlysubjected to temperatures of the combustion zone and the coarse' ore isconverted to a relatively porous trangible-y condition and partiallyroasted, passing through said zone a gas stream containing oxygen inamount less than that needed to support complete oxidation of saidsmalls and in quantityrestricted to maintain such temperaturesnot'substantially in excess of about l550 1"., subjecting the coarse orewhile in said 'zone to attrition suilicient to reduce the same to nes,maintaining the velocity oi' the gas stream passing through said zone-high enough but not substantially in excess of that. re-

quired to sweep out of said zone lines resulting produce sulfur dioxide,and withdrawing sulfur dioxide from' the upper end of said secondcombustion zone. f

